The present invention relates to a friction clutch used in, for example, a coupling device of a vehicle.
A typical friction clutch includes a drive clutch plate and a driven clutch plate which friction-engage with each other to transmit power.
Surface treatment is applied onto sliding surfaces of both clutch plates to retard wear by friction. The surface treatment includes, for example, nitride treatment, or quenching and tempering treatment. Wear of the sliding surfaces is reduced by reinforcing the sliding surfaces of both clutch plates by these surface treatments.
However, even in the case in which the surface treatment as describe above is applied, if both clutch plates are friction-engaged in the state without lubricant oil, or if a great power is transmitted even in the friction engagement with the lubricant oil, the sliding surfaces wear significantly, and the clutch plates have less durability.